I defined a function ProfitRetailSolo, and one of the parameters it takes is R.

I can not get closed form expressions for the value of R that maximizes ProfitRetailSolo. SO I am doing some numerical analysis.

As can be seen below I used Reduce and required the first deriviative to equal 0 and the second derivative to be negative.
With the specific values I am using for the other parameters- The output is that there are two values of R for which above holds: R == 73.1497 || R == 77.5785 (see below)

But when I then try to plug back above values in the derivative-- I dont get 0.
Also plotting the ProfitRetailSolo function, it does not have a maximum around R=73.
Can someone help me get correct result for the value of R that maximizes ProfitRetailSolo?